Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer element.
A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point in the region of interest, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel is based on the intensity of the echo return from the biological tissue being scanned. The outputs of the receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as a B-mode image of the anatomy being scanned.
In conventional B-mode imaging, image quality is determined largely by the point resolution which can be characterized by the point spread function (PSF) of the imager. The axial profile of the PSF can be sharpened by using short transmit bursts (higher frequency or fewer cycles) and/or pre-skewing of the transmit waveform to counteract tissue attenuation effects. The lateral dimension of the PSF can be reduced by using lower F-number (focal length to aperture ratio) and/or higher transmit frequency. In addition, all three dimensions of the PSF can be sharpened by using the second (or higher) harmonic frequency band on receive to form the image.
An increased lateral resolution, however, is often achieved at the expense of the acoustic frame rate for two reasons. First, the larger lateral spatial bandwidth (narrower PSF) must be accompanied by an appropriate increase in vector density (decrease in vector spacing) in order to satisfy spatial sampling requirements. Otherwise the expected improvements in lateral resolution will not actually be realized; instead, distracting lateral spatial aliasing artifacts may show up in the B-mode image. In non-zoom mode, increasing the vector density will generally compromise frame rate. Second, if the lateral resolution in the focal region is achieved by using lower F-numbers (larger apertures), the depth of field (axial length of the focal region) will be reduced. This means that more focal zones must be used in order to maintain acceptable image uniformity from the near field to the far field. Increasing the number of focal zones will also reduce the frame rate.
For live scanning of moving body parts, the acoustic frame rate must be maintained at some minimum acceptable level. Therefore, in practice, frame rate requirements tend to limit the maximum allowable vector density and number of focal zones, which in turn may limit the maximum aperture size and resolution that the system is capable of supporting. In prior art imagers, maximum resolution and high frame rates cannot generally be achieved simultaneously, except in zoom mode or by reducing the image wedge size to a very small area.